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Streams
and Watersheds - Stream Mechanics
Stream
hydrology
-- the study of water as it moves through stream channels
-- is a complicated science. Fortunately, for our purposes
this complexity can be simplified into a few basic concepts.
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channels
-- the "pipelines" through which water flows -- are
formed and maintained by the amount of water and sediment they
are required to carry during the average bankfull
flow, an event which normally occurs about every two years.
In a stable stream the amounts of water and sediment that come
into the channel on a regular basis are roughly equal to the
amounts of both that leave it (Figure 1-6). |
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Figure
1-6
Stable streams balance the amount of water and sediment
they carry. |
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Figure
1-7
Having downcut and then eroded laterally, this stream
is now healing as it develops perennial vegetation on
new banks. |
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When
either water or sediment load changes, the stream is said
to be "out of balance" and "trying to redefine
itself" by creating a new shape which will accommodate
the new conditions (Fig. 1-7).
An
increase in upland or streamside erosion can result in more
sediment than the stream can carry. One result might be a
raising of the stream bed (aggradation) and the creation of
a wider and shallower channel.
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Conversely,
the natural shape or slope of the channel might be modified
(for example, by human activity such as channel straightening
or the installation of riprap) so that the stream's water
flow and sediment load are being confined into a channel that
was not designed to handle them. If the same amount of water
is still flowing into the channel from upstream, the stream
may respond to this alteration by downcutting, because the
flow is faster and hence cuts into the channel bottom. Or
it may try to reestablish its old meander pattern, which will
result in lateral instability (bank erosion) as water eats
away at the streambanks. Either of these developments may
increase the amount of sediment that enters the stream as
well as causing other changes downstream or upstream.
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